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Electric-field-controlled magnetism can boost energy efficiency in widespread applications. However, technologically, this effect is facing important challenges: mechanical failure in strain-mediated piezoelectric/magnetostrictive devices, dearth of room-temperature multiferroics, or stringent thickness limitations in electrically charged metallic films. Voltage-driven ionic motion (magneto-ionics) circumvents most of these drawbacks while exhibiting interesting magnetoelectric phenomena. Nevertheless, magneto-ionics typically requires heat treatments and multicomponent heterostructures. Here we report on the electrolyte-gated and defect-mediated O and Co transport in a Co3O4 single layer which allows for room-temperature voltage-controlled ON-OFF ferromagnetism (magnetic switch) via internal reduction/oxidation processes. Negative voltages partially reduce Co3O4 to Co (ferromagnetism: ON), resulting in graded films including Co- and O-rich areas. Positive bias oxidizes Co back to Co3O4 (paramagnetism: OFF). This electric-field-induced atomic-scale reconfiguration process is compositionally, structurally, and magnetically reversible and self-sustained, since no oxygen source other than the Co3O4 itself is required. This process could lead to electric-field-controlled device concepts for spintronics., Financial support by the European Research Council (SPINPORICS 2014-Consolidator Grant, Agreement No. 648454), the Spanish Government (Projects MAT2017-86357-C3-1-R and associated FEDER), the Generalitat de Catalunya (2017-SGR-292) and the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Grant Agreement No. 665919 is acknowledged. E.P. is grateful to MINECO for the “Ramon y Cajal” contract (RYC-2012-10839). The ICN2 is funded by the CERCA programme/Generalitat de Catalunya. ICN2 also acknowledges the support from the Severo Ochoa Program (MINECO, Grant SEV-2013-0295). Work at UCD is supported by the US NSF (DMR1610060 and ECCS-1611424).

A synergetic approach to enhance magnetoelectric effects (i.e., control of magnetism with voltage) and improve energy efficiency in magnetically actuated devices is presented. The investigated material consists of an ordered array of Co–Pt microdisks, in which nanoporosity and partial oxidation are introduced during the synthetic procedure to synergetically boost the effects of electric field. The microdisks are grown by electrodeposition from an electrolyte containing an amphiphilic polymeric surfactant. The bath formulation is designed to favor the incorporation of oxygen in the form of cobalt oxide. A pronounced reduction of coercivity (88%) and a remarkable increase of Kerr signal amplitude (60%) are observed at room temperature upon subjecting the microdisks to negative voltages through an electrical double layer. These large voltage-induced changes in the magnetic properties of the microdisks are due to (i) the high surface-area-to-volume ratio with ultranarrow pore walls (sub-10 nm) that promote enhanced electric charge accumulation and (ii) magneto-ionic effects, where voltage-driven O2– migration promotes a partial reduction of CoO to Co at room temperature. This simple and versatile procedure to fabricate patterned “nano-in-micro” magnetic motifs with adjustable voltage-driven magnetic properties is very appealing for energy-efficient magnetic recording systems and other magnetoelectronic devices., This work was funded by the European Research Council under the SPIN-PORICS 2014-Consolidator Grant (Agreement No. 648454), the Generalitat de Catalunya (2017-SGR292 project), the Spanish Government (MAT2017-86357-C3-1-R, MAT2014-57960-C3-1-R, and associated FEDER projects), and the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No. 665919. J.F. and E.P. acknowledge the Juan de la Cierva (IJCI-2015-27030) and Ramon y Cajal (RYC-2012-10839) fellowships, respectively, from MINECO. The ICN2 is funded by the CERCA program/Generalitat de Catalunya. ICN2 also acknowledges the support from the Severo Ochoa Program (MINECO, grant SEV-2013-0295)